The present invention relates to an optically transparent film especially used in a protective film (hereafter including a material expressed as a xe2x80x9clayerxe2x80x9d) for optical disks, more specifically to an optically transparent film that can decrease the number of particles formed during the formation of a film with sputtering, that has a high transmittance in the visible region of the formed film, and that is suitable for an optical disk having a low reflectance, in particular for a phase-change type optical disk; and a sputtering target for forming such a film.
In recent years, high-density optical disks have rapidly attracted public interests because they can record and play without the need of magnetic heads.
These optical disks are classified into three types: the read-only type, the write-once type, and the rewritable type, and the phase-change system used in the write-once type and the rewritable type has particularly attracted public attention. The principle of writing and reading using such a phase-change type optical disk will be described briefly below.
A phase-change type optical disk is used for writing and reading information by heating the recording layer on a substrate by the irradiation of laser beams, and causing crystallographic phase change (between amorphous and crystalline phases) to occur in the structure of the recording layer. More specifically, information is read by sensing change in reflectance caused by change in optical constants between the phases.
The above-described phase change is caused by the irradiation of laser beams narrowed down to a diameter of 1 to several microns. In this case, for example, if laser beams of a diameter of 1 xcexcm pass at a linear velocity of 10 m/s, the time that a certain spot on the optical disk is irradiated is 100 ns, in which the above-described phase change and reflectance must be sensed.
For realizing the above-described crystallographic phase change, i.e. phase change between amorphous and crystalline phases, the heat of melting and quenching is imparted not only to the phase-change recording layer of the optical disk, but also to the surrounding protective film and the reflective film of an aluminum alloy repeatedly.
From these facts, as FIG. 1 shows, the phase-change optical disk has a four-layer structure in which a Gexe2x80x94Sbxe2x80x94Te-based recording thin film 4 or the like is sandwiched by protective films 3 and 5 of a ZnSxe2x80x94SiO2-based high-melting-point dielectric, and further a reflective film 6 of an aluminum alloy is disposed.
Among these, the reflective film 6 and protective films 3, 5 is required to have optical functions to increase the absorption of amorphous and crystalline portions, and to increase difference in reflectance, as well as the function to prevent the distortion of the recording film 4 due to damp-proof or heat, and the function to control the thermal conditions on recording (see xe2x80x9cKogaku (Optics),xe2x80x9d Vol. 26, No. 1, pp. 9-15).
As described above, the high melting-point protective films 3, 5 must be resistant to repetitive thermal stress of heating and cooling, the reflective film and other components must not be affected by these thermal effects, and the protective films themselves must be thin and low reflective, and must not be deteriorated. In this sense, the protective films 3, 5 play an important role.
In FIG. 1, the symbol 1 represents the laser incident direction, 2 represents a substrate of polycarbonate or the like, 7 represents an overcoat, and 8 represents an adhesive layer.
The above-described protective films 3 and 5 are normally formed by the sputtering method. In this sputtering method, a positive electrode and a target consisting of a negative electrode is made to face to each other, and a high voltage is input between the substrate and the target in an inert gas atmosphere to create an electric field. The method uses the principle in which electrons impinge against the inert gas to form plasma, anions in the plasma impinge on the surface of the target (negative electrode) to knock on atoms constituting the target, and these knock-on atoms are deposited on the facing surface of the substrate to form a film.
As the target for forming the above-described protective films, a ZnSxe2x80x94SiO2 sputtering target, manufactured by sintering the mixed powder of SiO2 powder and ZnS powder, has been used.
In the stage of forming thin films by sputtering using a ZnSxe2x80x94SiO2 sputtering target, when the quantity of coating exceed a certain level, cluster-like coarse grains, known as particles, are deposited on the thin film. The main cause of the formation of these particles is that the mist produced by sputtering deposit on the walls of the sputtering chamber and on various apparatuses, it is peeled off as debris when the quantity exceeds a certain level, the debris float within the sputtering chamber, and deposit again on the substrate or the thin film.
Since these particles degrade the properties of the thin film, sputtering must be once suspended in the stage when a large quantity of the particles deposit on the substrate or the thin film, and the sputtering chamber must be opened to remove deposits of the film that cause particles to be formed on the walls of the sputtering chamber and on various apparatuses.
This lowers productivity significantly. Although the reason why these deposits of the film adhere on the walls of the sputtering chamber and on various apparatuses has not been well known, the cause-result relationship has been estimated in the manufacturing process of the ZnSxe2x80x94SiO2 target, specifically, the mixing and sintering steps of SiO2 powder and ZnS powder. However, the solution more than the estimation has not been found.
Also, although the protective films formed by sputtering have been required to have a reflectance as low as possible, the feasibility of the improvement in the steps of the manufacturing process of the ZnSxe2x80x94SiO2 target has not been sufficiently studied.
Especially, the large problem of the above-described conventional ZnSxe2x80x94SiO2 target is that direct current sputtering cannot be performed because these materials are insulators. Therefore, low-efficiency methods such as high frequency (RF) sputtering must be used. These methods require sputtering for a long time to obtain a desired thickness, and raise an unfavorable problem that the number of particles, which must be decreased, are rather increased.
The present invention solves the above problems by fundamentally reviewing sputtering target materials to reduce the formation of particles as much as possible, to decrease the frequency of interruption or discontinuance of sputtering to improve production efficiency, and to obtain a protective film for optical disks having a large transmittance and a low reflectance.
According to the present invention, there are provided 1) an optically transparent film containing 0.01 to 20% by weight one or more glass forming oxide selected from a group consisting of Nb2O5, V2O5, B2O3, SiO2, and P2O5, 0.01 to 20% by weight Al2O3 or Ga2O3; balance being ZnO; 2) the optically transparent film according to the above-described 1), further containing 0.01 to 5% by weight hard oxide of ZrO2 and/or TiO2; 3) a sputtering target for forming an optically transparent film containing 0.01 to 20% by weight one or more glass forming oxide selected from a group consisting of Nb2O5, V2O5, B2O3, SiO2, and P2O5; 0.01 to 20% by weight Al2O3 or Ga2O3; balance being ZnO; and 4) the sputtering target for forming an optically transparent film according to the above-described 3), further containing 0.01 to 5% by weight hard oxide of ZrO2 and/or TiO2.